Cryosurgery has received increased attention as an alternative to conventional surgical techniques. Instead of using a blade to cut tissues, a surgical device with a cold tip or surface is used to cool the affected tissue to a sufficiently low temperature to ablate the tissue. For example, cryosurgical ablation is often performed to correct arrhythmia, irregular heart beat, by ablating specific regions of the heart or of veins entering the heart from which erroneous contraction impulses originate. Cryosurgery is also used extensively in dermatology, to remove abnormal tissue with less scarring than is possible using conventional surgery. Since application of a cold surface to tissues deadens the nerve endings where the tissue is removed, cryosurgery results in less pain and faster patient recovery.
Devices for performing cryosurgery vary in shape and method of use. In the case of cardiac tissue ablation, a catheter is typically introduced through the vascular system to a precise location of tissue to be treated. The tip of the catheter is then cooled to a temperature sufficiently low to ablate the tissue. These devices must be capable of cooling the operative surface to a temperature sufficiently low so that the tissue placed in contact with that surface will freeze and be destroyed. However, the diseased or degenerated tissue that must be destroyed is generally in close proximity to non-targeted, healthy tissue that should be preserved. This places complex demands on the shape, flexibility and guidance of the probe or catheter used for cryoablation, since it must be able to reach the affected area, and apply the cold selectively to the selected portion of tissue without injuring any other tissue which may be in contact with or proximity to the rest of the device.
Additional difficulties exist when the cryoablation surface is at the distal end of a catheter that is introduced into a patient's blood vessels. To avoid damaging the non-targeted tissues, the catheter cannot be cooled before the tip has reached the target tissue. Once the tip has reached the target tissue, the cooling arrangement must lower the temperature of the operative surface(s) of the tip rapidly, and must maintain the lowered temperature despite heating loads applied to the device until the desired tissue has been ablated. For example, cooling must overcome the warming effect on the catheter and the target tissue of the flow of blood through vessels in which the catheter is located. Rapid cooling of the device is complicated by the fact that the cooling surfaces may be placed a significant distance from the point at which the catheter enters the patients body. In addition, only a small portion of the device must be cooled, while the remainder must remain at a substantially higher temperature than the tip to avoid damaging non-targeted tissue.